Research: partial migration

Under what conditions should individuals of migratory species skip migration and how frequently?

In many migratory species, only some individuals migrate each year (termed 'partial migration'). Most theoretical work on partial migration focuses on species where individuals migrate between breeding and wintering grounds. In these species, the decision to migrate or stay resident is based on tradeoffs between survival and competition. However, partial migration is also common in species where individuals migrate between feeding and breeding grounds. In these species, migration is tied to reproduction, so the decision to migrate or stay is based on tradeoffs between current and future reproduction. We developed the first set of models based on this second type of partial migration. We find that partial migration is expected to occur more frequently as the mortality cost of migration increases, and when there is a fecundity benefit to storing energy over several years before reproducing. Stochastic environments can also favor partial migration. (Shaw & Levin 2011)

These same models can also be used to understand breeding frequency in any species with 'intermittent breeding' where individuals wait several years between breeding attempts (Shaw & Levin 2013).

We also show that for a species of land crab with partial migration, global climate change (based on projections) will likely reduce the frequency at which individuals migrate, which could have quite a negative impact for this species (Shaw & Kelly 2013).

We show that migration to recover from infection is another mechanism that can favor partial migration, for species that are long-lived with relatively low rates of infection (Shaw & Binning 2016).